Switching power supply unit

ABSTRACT

A switching power supply unit of the present invention a timing generating circuit ( 121 ) which receives a first control signal formed by a rectifier-transistor driving circuit ( 104 ), forms a second control signal based on the first control signal, and supplies the second control signal to a control electrode of the rectifier transistor ( 113 ).  
     The first control signal is synchronized with the switching operation of a half-bridge circuit ( 102 ), and the second control signal exceeds a threshold voltage of a rectifier transistor ( 113 ) at a timing substantially equal to the timing that one edge of the first control signal is generated and falls below the threshold voltage of the rectifier transistor ( 113 ) at a timing earlier by predetermined time than the timing that the other edge of the first control signal is generated.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a switching power supply unit,and more particularly to a synchronous rectifying switching power supplyunit, a switching power supply unit having a half-bridge circuit, and aswitching power supply unit using a plurality of converters connected inseries.

[0002] (Related Art 1)

[0003] Conventionally, DC/DC converters are known as a switching powersupply unit. A typical DC/DC converter converts an alternating currentinput to direct current once by using a switching circuit, thentransforms (boosting or lowering) the voltage by using a transformer,and further converts the direct current to alternating current by usingan output circuit, whereby alternating current output having voltagedifferent from the input voltage can be obtained.

[0004] In some cases, switching elements such as transistors areemployed in an output rectifier for use in the DC/DC converter, so thatthe switching elements may be synchronously controlled with theswitching circuit on the input side. The DC/DC converter having such anoutput rectifier is generally called as a synchronous rectifyingswitching power supply unit.

[0005]FIG. 15 is a circuit diagram of a conventional synchronousrectifying switching power supply unit.

[0006] As shown in FIG. 15, the conventional switching power supply unitincludes a transformer 1, a half-bridge circuit 2 provided on theprimary side of the transformer 1, a rectifier circuit 3 provided on thesecondary side of the transformer 1, a rectifier-transistor drivingcircuit 4 provided on the secondary side of the transformer 1, asmoothing circuit 5 provided at the following stage of the rectifiercircuit 3, and a control circuit 9 for controlling on/off of a firstmain switch 7 and a second main switch 8 provided in the half-bridgecircuit 2 based on the result of monitoring output voltage Vo via aninsulating circuit 6.

[0007] The half-bridge circuit 2 includes a first input capacitor 11 anda second input capacitor 12 connected in series between both ends of aninput power supply 10 in addition to the first and second main switches7 and 8. The primary winding 20 of the transformer 1 is connectedbetween a node where the first and second main switches 7 and 8 arejoined and a node where the first and second input capacitors 11 and 12are joined. The rectifier circuit 3 has a first rectifier transistor 13and a second rectifier transistor 14. The drain of the first rectifiertransistor 13 is connected to the first secondary winding 21 of thetransformer 1, whereas the drain of the second rectifier transistor 14is connected to the second secondary winding 22 of the transformer 1. Asshown in FIG. 15, as the source of the first rectifier transistor 13 andthe source of the second rectifier transistor 14 are short-circuited, avoltage waveform appearing between the common source node of bothtransistors and a node where the first and second secondary windings 21and 22 of the transformer 1 are joined forms an output from therectifier circuit 3. The rectifier-transistor driving circuit 4 has afirst diode 15 connected between the gate and source of the secondrectifier transistor 14 and a second diode 16 connected between the gateand source of the first rectifier transistor 13. The third secondarywinding 23 of the transformer 1 is connected between the cathode of thefirst diode 15 and the cathode of the second diode 16. Further, thesmoothing circuit 5 has a smoothing inductor 17 and a smoothingcapacitor 18.

[0008] With the arrangement above, the first and second main switches 7and 8 are turned on alternately under the control of the control circuit9 at intervals of predetermined dead time, whereby the output voltage Vodetermined by the input voltage Vin and the turn ratio of thetransformer 1 is applied to a load 19.

[0009]FIG. 16 is a timing chart showing the operation of theconventional synchronous rectifying switching power supply unit. In FIG.16, Vgs7 and Vgs8 mean the gate-source voltages of the first and secondmain switches 7 and 8 respectively; Vds13 and Vds14 mean thesource-drain voltages of the first and second rectifier transistors 13and 14 respectively; and Vgs13 and Vgs14 means the gate-source voltagesof the first and second rectifier transistors 13 and 14 respectively.

[0010] As shown in FIG. 16, in the conventional synchronous rectifyingswitching power supply unit, the first and second main switches 7 and 8are driven alternately under the control of the control circuit 9 atintervals of predetermined dead time, and in response to the operation,the secondary voltage is generated across the source and drain of thesecond rectifier transistor 14 during the interval the first main switch7 is “on”, whereas the secondary voltage is generated across the sourceand drain of the first rectifier transistor 13 during the interval thesecond main switch 8 is “on”.

[0011] In this case, in the rectifier-transistor driving circuit 4, thefirst diode 15 is turned on during the interval the first main switch 7is “on” and the second diode 16 is turned on during the interval thesecond main switch 8 is “on”. Consequently, during the interval thefirst main switch 7 is “on”, the gate-source channel of the firstrectifier transistor 13 is driven and turned on, and during the intervalthe second main switch 8 is “on”, the gate-source channel of the secondrectifier transistor 14 is driven and turned on. Further, as the gate ofthe first rectifier transistor 13 and the gate of the second rectifiertransistor 14 are short-circuited via the third secondary winding 23 ofthe transformer 1 during the interval the first and second main switches7 and 8 both are “off”, the gate-source voltages of the first rectifiertransistor 13 and the second rectifier transistor 14 each becomeintermediate voltages.

[0012] As the first rectifier transistor 13 is turned on during thewhole interval the second main switch 8 is “off” and as the secondrectifier transistor 14 is turned on during the whole interval the firstmain switch 7 is “off”, no current is practically allowed to flow intothe body diode of the first rectifier transistor 13 and the body diodeof the second rectifier transistor 14, so that rectification can becarried out with a small loss.

[0013] (Related Art 2)

[0014] There have been proposed so-called two-stage converters forelectronic systems like computers and as one example of a switchingpower supply unit for efficiently and stably supplying voltage, apreceding-stage buck converter and a following-stage half-bridgeconverter are combined in such a two-stage converter.

[0015] The buck converter is used for stepping down input voltage to acertain voltage level, whereas the half-bridge converter employs ahalf-bridge circuit for converting the input voltage to AC voltage,insulating, rectifying and smoothing the AC voltage to generate DCvoltage.

[0016] A rectifying-smoothing circuit comprises a self-drive typesynchronous rectifying circuit formed with a synchronous rectifyingswitch element connected to the secondary winding side of a transformer,capacitors and an inductor.

[0017] As described in a document under the title of “Buck +Halfbridge(d=50%) Topology Applied to very Low Voltage Power Converters” by P.Alou, J. Oliver, J. A. Cobos, O. Garcia and J. Uceda in the IEEE AppliedPower Electronics Conference (APEC), 2001, a two-stage converterarrangement is made through the steps of fixing to 50% the duty ratio ofamain switch element provided in a following-stage half-bridge converterand controlling the duty ratio of a switching element provided in apreceding-stage buck converter so as to make the duty ratio of theswitching element variable in accordance with output voltage.

[0018] (Related Art 3)

[0019] There has been proposed a technique recently for exciting theprimary winding of a transformer by using a half-bridge circuit, whereina buck converter circuit and the half-bridge circuit are connected inseries as the primary circuit of a switching power supply unit; and thebuck converter circuit is used to step down input voltage Vin and supplythe input voltage thus stepped down to the half-bridge circuit(Buck+Half Bridge (d=50%) Topology applied to Very Low Voltage PowerConverters, IEEE APEC, 2001, Session 19.4).

[0020] When these circuits above are used as the primary circuit of theswitching power supply unit, control is exerted so that the duty of aswitching element provided in the half-bridge circuit is fixed to apredetermined quantity and that the duty of a switching element providedin the buck converter circuit is set to a predetermined quantityaccording to output voltage Vo. As comparatively low voltage is thusobtainable efficiently and stably as the output voltage Vo, thisswitching power supply unit is most suitable usable as a power supplyfor computers, for example.

[0021]FIG. 17 is a circuit diagram of a conventional switching powersupply unit having such a primary circuit as described above.

[0022] As shown in FIG. 17, the conventional switching power supply unitincludes a transformer 51, a buck converter circuit 53 connected to aninput power supply 52, a half-bridge circuit 54 that is connected to thebuck converter circuit 53 and used for exciting the primary winding ofthe transformer 51, a rectifier circuit 55 provided on the secondaryside of the transformer 51, a smoothing circuit 57 provided at thefollowing stage of the rectifier circuit 55 and connected to a load 56,and a control circuit 63 for monitoring output voltage Vo via aninsulating circuit 58 and performing on/off control over a first and asecond main switch 59 and 60 provided in the buck converter circuit 53according to the monitored result and performing on/off control over athird and a fourth main switch 61 and 62 provided in the half-bridgecircuit 54.

[0023] The buck converter circuit 53 has an inductor 64 in addition tothe first and second main switches 59 and 60; the half-bridge circuit 54has a first and a second input capacitor 65 and 66 connected in seriesacross the output terminal of the buck converter circuit 53 in additionto the third and fourth main switches 61 and 62; and the primary windingof the transformer 51 is connected between a node where the third andfourth main switches 61 and 62 are joined and a node where the first andsecond input capacitors 65 and 66 are joined. Further, the rectifiercircuit 55 has a first and a second diode 67 and 68; and the smoothingcircuit 57 has a smoothing inductor 69 and a smoothing capacitor 70. Therectifier circuit 55 and the smoothing circuit 57 constitute an outputcircuit.

[0024] With the arrangement above, the first and second main switches 59and 60 provided in the buck converter circuit 53 are alternately turnedon with predetermined dead time held therebetween under control of thecontrol circuit 63, whereby the constant internal voltage Vin2determined by the duties of input voltage Vin1 and the first and secondmain switches 59 and 60 appears across the output terminal of the buckconverter circuit 53. On the other hand, the third and fourth mainswitches 61 and 62 provided in the half-bridge circuit 54 arealternately turned on/off with a predetermined quantity of duty undercontrol of the control circuit 63. Thus, the constant output voltage Vodetermined by the internal voltage Vin2 and the turn ratio of thetransformer 51 is given across the load 56.

[0025] Regarding the first related art, what has been described aboverefers to ideal operation, and in actual circuits, there unavoidablyoccurs a slight delay in the timing of operations of the first rectifiertransistor 13 and the second rectifier transistor 14. Ideally, the firstrectifier transistor 13 is turned off simultaneously at the timing (timet0) the secondary voltage is generated across the source and drain ofthe secondary rectifier transistor 13, and the second rectifiertransistor 14 is turned off simultaneously at the timing (time t1) thesecondary voltage is generated across the source and drain of thesecondary rectifier transistor 14. Actually, however, the timing thefirst rectifier transistor 13 is turned off slightly delays behind thetime t0 and the timing the second rectifier transistor 14 is turned offslightly delays behind the time t1.

[0026] For the reason above, a through current flows into the firstrectifier transistor 13 in a brief interval of time after the secondaryvoltage is generated across the source and drain of the first rectifiertransistor 13, and similarly, through current flows into the secondrectifier transistor 14 in a brief interval of time after the secondaryvoltage is generated across the source and drain of the second rectifiertransistor 14. The through currents result in power loss and the problemis that the lowering of conversion efficiency is caused to the wholeswitching power supply unit.

[0027] In the two-stage converter as described in the second relatedart, current flowing through a synchronous rectifying switch element onthe secondary winding side of a transformer is caused to have acommutation period due to the leakage inductance of the transformerprovided in the half-bridge circuit and then voltage is generated acrossboth ends of the synchronous rectifying switch element.

[0028] In case where the commutation period is longer than delay in theoperation of the synchronous rectifying switch element (turn on/offperiod), through current flows as the synchronous rectifying switchelements are simultaneously turned on and the synchronous rectifyingswitch elements may be damaged when the worst comes to the worst.

[0029] Particularly in the case of a low ON resistant synchronousrectifying switch element, operation-delay time tends to become longer,this phenomenon appears conspicuously.

[0030] Although this problem can be dealt with by coarsely coupling thetransformer in order to increase the leakage inductance and prolong thecommutation period. However, power loss may increase caused by theincrease of the interval that the synchronous rectifying switch elementcannot be turned on, and further, there may be brought about a badinfluence resulting from an increase in loss because of the leakageinductance and spike noise.

[0031] The synchronous rectifying switch elements can be prevented frombeing simultaneously turned on by adding to the half-bridge circuit adrive timing circuit for controlling the timing that the synchronousrectifying switch element is operated. However, the problem in this caseis that a switching power supply unit tends to become large-sizedaccompanied with an increase in cost as the number of parts increases.

[0032] Regarding the third related art, a user may be requested to beable to switch the values of the output voltage Vo in order to havedifferent kinds of loads driven by one kind of switching power supplyunit. In case where the user is allowed to switch the output voltage Vobetween 3.3V and 1.5V, a step-down range to be covered by the buckconverter circuit 3 as the first stage converter grows larger, and theload of the buck converter circuit 3 is heavy when lower voltage (e.g.,1.5V) is required as the output voltage Vo, and the problem is that losstends to increase.

[0033] In case where a lower value of the output voltage Vo is set bythe user, the output voltage Vo can be lowered by reducing not only theduty of the buck converter circuit 3 but also the duty of thehalf-bridge circuit 4 as a second stage converter. In this case,however, the stability of the output voltage Vo may be ruined because aplurality of converters operate to stabilize the output voltage Vo. Inorder to prevent the stability of the output voltage Vo from beingruined, the converter-to-converter operation needs to be properlyregulated, which results in complicating the control operation.Particularly when transistors as rectifying elements constituting arectifier circuit are used and turned on/off by utilizing the secondaryvoltage of the transformer 1, the loss produced in the rectifier circuittends to increase because the dead time of the half-bridge circuit 4fluctuates as the duty of the half-bridge circuit 4 fluctuates. In casewhere the duty of the half-bridge circuit 4 is lowered in response to ademand for lower voltage (e.g., 1.5V) as the output voltage Vo, the deadtime of the half-bridge circuit 4 increases, whereby the interval duringwhich no voltage is generated on the secondary side of the transformer 1becomes longer. Consequently, as the conducting period of the rectifiertransistors constituting the rectifier circuit becomes shortened,current is allowed to flow into the body diodes over a long period oftime.

SUMMARY OF THE INVENTION

[0034] It is therefore an object of the invention to provide a switchingpower supply unit adapted to effectively prevent the generation ofthrough currents.

[0035] Another object of the invention is to provide a switching powersupply unit capable of substantially improving the reliability of aself-drive type secondary rectifier circuit and also forming a highlyefficient power supply at low cost.

[0036] Still another object of the invention is to provide a switchingpower supply unit which uses a plurality of converters connected inseries and is capable of properly switching output voltage Vo from ahigh level to a low level.

[0037] Another object of the invention is to provide a switching powersupply unit which uses a plurality of converters connected in series andis capable of switching output voltage Vo from a high level to a lowerunder a simple type of control.

[0038] Still another object of the invention is to provide a switchingpower supply unit which uses a plurality of converters connected inseries and is capable of switching output voltage Vo from a high levelto a lower level while suppressing an increase in the loss generated ina rectifier circuit.

[0039] The object of the invention is accomplished by a switching powersupply unit comprising a transformer, a switching circuit provided onthe primary side of the transformer, a synchronous rectifier circuitwhich is provided on the secondary side of the transformer and has atleast a rectifier transistor, a rectifier-transistor driving circuitwhich is provided on the secondary side of the transformer and forms afirst control signal synchronous with the switching operation of theswitching circuit, and a timing generating circuit for forming a secondcontrol signal which exceeds the threshold voltage of the rectifiertransistor at a timing substantially equal to the timing one edge of thefirst control signal is generated on receiving the first control signaland which falls below the threshold voltage of the rectifier transistorat a timing earlier by predetermined time than the timing the other edgeof the first control signal is generated and for supplying the resultingsecond control signal to the control electrode of the rectifiertransistor.

[0040] As the off timing of rectifier transistor is hastened by thetiming generating circuit according to the invention, the generation ofthrough currents can effectively be prevented, whereby the conversionefficiency of the whole switching power supply unit is enhanced becausepower loss is reducible.

[0041] According to an embodiment of the invention, the waveform of thefirst control signal is a waveform alternately repeating a firstpotential, a second potential and an intermediate potential inserted inbetween the first and second potentials, the one edge of the firstcontrol signal being defined by the timing the one edge varies from thefirst potential to the intermediate potential, the other edge of thefirst control signal being defined by the timing the other edge variesfrom the intermediate potential to the first potential.

[0042] According to another embodiment of the invention, during theinterval the first control signal varies from the intermediate potentialto the first potential after the first control signal varies from thesecond potential to the intermediate potential, the voltage of thesecond control signal falls below the threshold voltage of the rectifiertransistor.

[0043] According to another embodiment of the invention, the timinggenerating circuit includes first means for, on receiving the firstcontrol signal, forming an intermediate signal which varies from a firstlogical level to a second logical level in response to the one edge ofthe first control signal and varies from the second logical level to thefirst logical level in response to the variation of the first controlsignal from the second potential to the intermediate potential; andsecond means for, on receiving the intermediate signal, forming thesecond control signal by providing a delay to the variation of theintermediate signal from the second logical level to the first logicallevel.

[0044] According to another embodiment of the invention, the first meansincludes a divider circuit for dividing the first control signal, adelay circuit for delaying the output signal of the divider circuit, anda comparator for comparing the first control signal with the outputsignal of the delay circuit whereby to form the intermediate signal.

[0045] According to another embodiment of the invention, the delaycircuit includes a first time-constant circuit for providing a delay tothe one-directional variation of the output signal of the dividercircuit, and a second. time-constant circuit for providing a delay tothe reverse-directional variation of the output signal of the dividercircuit.

[0046] According to another embodiment of the invention, the timeconstant of the first time-constant circuit is set so that the potentialof the output signal of the delay circuit rises above at least theintermediate potential at the timing the first control signal variesfrom the second potential to the intermediate potential and wherein thetime constant of the second time-constant circuit is set so that thepotential of the output signal of the delay circuit falls below at leastthe intermediate potential at the timing the first edge of the firstcontrol signal is generated.

[0047] According to another embodiment of the invention, the switchingcircuit is one selected from a half-bridge circuit, a full-bridgecircuit, a push-pull circuit and an active clamping circuit.

[0048] The object of the invention is accomplished further by theswitching power supply unit comprising a switching circuit which isconnected to an input power supply and has a first and a second mainswitch which alternately conduct at intervals of dead time, a firstrectifier transistor for performing rectifying operation during theinterval the second main switch remains non-conducting and a secondrectifier transistor for performing rectifying operation during theinterval the first main switch remains non-conducting, and means fordriving the first and second rectifier transistors, wherein the means isused to supply an ON signal to the control electrode of the firstrectifier transistor over the substantially whole period of first deadtime to be inserted and also to supply the ON signal to the electrode ofthe second rectifier transistor for a part of period of the first deadtime when the conducting main switch is switched from the second mainswitch to the first main switch and wherein the means is used to supplythe ON signal to the control electrode of the second rectifiertransistor over the substantially whole period of second dead time to beinserted and also to supply the ON signal to the electrode of the firstrectifier transistor for a part of period of the second dead time whenthe conducting main switch is switched from the first main switch to thesecond main switch.

[0049] Even according to this invention, the generation of throughcurrents can effectively be prevented, whereby the conversion efficiencyof the whole switching power supply unit is enhanced because power lossis reducible.

[0050] According to an embodiment of the invention, a part of period ofthe first dead time is a consecutive period including the timing ofstarting the first dead time and wherein a part of period of the seconddead time is a consecutive period including the timing of starting thesecond dead time.

[0051] In order to solve the foregoing problems, a switching powersupply unit according to the invention includes a first and a secondswitching element which are provided on the primary winding side of atransformer and connected to a power supply in series, a converterhaving a first and a second synchronous rectifying switch element whichare connected to the secondary winding side of the transformer inseries, and a driving circuit for controlling the operation of the firstand second switching elements and generating a first and a secondcontrol signal having a dead time period in which the first and secondswitching elements are not conducting.

[0052] According to the invention, since the first and secondsynchronous rectifying switch elements can surely be prevented frombeing simultaneously turned on and since the commutation derived fromthe leakage inductance of the transformer can be controlled in anoptimum manner, it becomes possible to materialize low-cost, reliableand low-cost switching power supply unit.

[0053] The object of the invention is accomplished by a switching powersupply unit including a transformer, a first and a second converterconnected between a supply input terminal and the primary winding of thetransformer in series, an output circuit connected to the secondarywinding of the transformer, and control circuits for controlling theoperation of the first and second converters, wherein the controlcircuits are used for controlling the first converter in terms of dutyand also controlling the second converter in terms of frequency.

[0054] According to the invention, the first converter is controlled interms of duty, whereas the second converter is controlled in terms offrequency, whereby even when switching of output voltage is requested bya user, the switching operation can simply be controlled.

[0055] According to an embodiment of the invention, the control circuitcontrols the duty of the first converter according to the present outputvoltage outputted from the output circuit and controls the operatingfrequency of the second converter according to the set value of thepresent output voltage, regardless of the present output voltage.

[0056] According to another embodiment of the invention, as the firstconverter and the second converter share their roles with each other, itis unnecessary to coordinate both the converters closely.

[0057] According to another embodiment of the invention, further, thecontrol circuit controls the second converter so that dead time is madeconstant, regardless of the operating frequency.

[0058] According to the another embodiment of the invention, further, aself-drive type synchronous rectifier circuit formed with rectifiertransistors is contained in the output circuit and wherein the dead timeis set substantially equal to an interval resulting from subtracting acommutation period due to leakage from the transformer from delay timein the operation of the rectifier transistors.

[0059] According to another embodiment of the invention, further, theloss generated in the output circuit can effectively be suppressed whilethrough current is prevented from being generated.

[0060] According to another embodiment of the invention, further, thefirst converter is a buck converter circuit and the second converter isa half-bridge circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

[0061]FIG. 1 is a circuit diagram of a switching power supply unitaccording to the first embodiment of the invention;

[0062]FIG. 2 is a circuit diagram of a first and a second timinggenerating circuits;

[0063]FIG. 3 is a timing chart showing the operation of the switchingpower supply unit;

[0064]FIG. 4 is a circuit diagram of a switching power supply unitwherein a full-bridge circuit is used as the primary circuit of thetransformer by way of example;

[0065]FIG. 5 is a circuit diagram of a switching power supply unitwherein a push-pull circuit is used as the primary circuit of thetransformer by way of example;

[0066]FIG. 6 is a circuit diagram of a switching power supply unitwherein an active clamping circuit is used as the primary circuit of thetransformer by way of example;

[0067]FIG. 7 is a circuit diagram of a switching power supply unitwherein other circuits are used as the secondary circuit of thetransformer;

[0068]FIG. 8 is a circuit diagram of a switching power supply unitaccording to the second embodiment of the invention;

[0069]FIG. 9 is a timing chart showing the voltage/current waveform ofeach portion in the switching power supply unit of FIG. 8 when the sumof a dead time period and leakage from a transformer is equal toswitching delay time of a synchronous rectifying element;

[0070]FIG. 10 is a timing chart showing the voltage/current waveform ofeach portion when the commutation period of leakage from the transformeris shorter than switching delay time of the synchronous rectifyingelement in the switching power supply unit studied by the presentinventor;

[0071]FIG. 11 is a timing chart showing the voltage/current waveform ofeach portion when the commutation period of leakage from the transformeris longer than switching delay time of the synchronous rectifyingelement in the switching power supply unit studied by the presentinventor;

[0072]FIG. 12 is a circuit diagram of a switching power supply unitaccording to another example in the second embodiment of the invention;

[0073]FIG. 13 is a circuit diagram of a switching power supply unitaccording to the third embodiment of the invention;

[0074]FIGS. 14A and 14B are timing charts illustrating the operation ofa control circuit;

[0075]FIG. 15 is a circuit diagram showing a conventional synchronousrectifying switching power supply unit of first related art;

[0076]FIG. 16 is a timing chart showing the operation of theconventional synchronous rectifying switching power supply unit of thefirst related art; and

[0077]FIG. 17 is a circuit diagram of a conventional switching powersupply unit of the third related art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0078] The preferred embodiments of the invention will now be describedwith reference to the accompanying drawings.

[0079] (Embodiment 1)

[0080]FIG. 1 is a circuit diagram of a switching power supply unit 120as the first embodiment of the invention.

[0081] As shown in FIG. 1, the switching power supply unit 120 accordingto this embodiment of the invention is a synchronous rectifyingswitching power supply unit of a so-called half-bridge type like atypical conventional switching power supply unit. However, the switchingpower supply unit 120 according to this embodiment of the invention isdifferent in that: a first timing generating circuit 121 is inserted inbetween a rectifier-transistor driving circuit 104 and the gate of afirst rectifier transistor 113, and a second timing generating circuit122 is inserted in between the rectifier-transistor driving circuit 104and the gate of a second rectifier transistor 114; and that a firstauxiliary capacitor 123 is connected between both ends of a first diode115, and a second auxiliary capacitor 124 is connected between both endsof a second diode 116. As the switching power supply unit according tothis embodiment of the invention is similar in construction to theconventional switching power supply unit shown in FIG. 15, thedescription of the component elements similar to the conventionalswitching power supply unit will be omitted.

[0082] The first and second timing generating circuits 121 and 122 arecircuits respectively provided with input terminals 136 connected to therectifier-transistor driving circuit 104 as well as output terminals 137connected to the corresponding rectifier transistors 113 and 114,wherein the waveform of the signal supplied to the input terminal 136 istransformed and the signal having the transformed waveform is outputtedfrom the output terminal 137. Further, the first and second auxiliarycapacitors 123 and 124 are capacitors for providing capacitanceequivalent to the gate-source capacitance of the first and secondrectifier transistors 13 and 14 of the switching power supply unit ofFIG. 115.

[0083]FIG. 2 is a circuit diagram of the first and second timinggenerating circuits 121 and 122.

[0084] As shown in FIG. 2, each of the first and second timinggenerating circuits has a comparator 125, resistors 126 to 129, diodes130 to 133 and capacitors 134 and 135. The non-inverted input terminal(+) of the comparator 125 is connected to the input terminal 136,whereas the inverted input terminal (−) thereof is connected via thediode 130 to a node where the resistors 126 and 127 are joined.

[0085] The resistors 126 and 127 are used to divide voltage V1 at theinput terminal 136 whereby to serve to supply the divided voltage to theinverted input terminal (−) of the comparator 125. The resistor 126, thediode 130 and the capacitor 134 work as a time constant circuit (a firsttime-constant circuit) when the voltage V1 at the input terminal 136changes from a low level to a high level. Moreover, the resistor 128,the diode 131 and the capacitor 134 work as a time constant circuit (asecond time-constant circuit) when the voltage V1 at the input terminale changes from the high level to the low level. Thus, voltage V2 at theinverted input terminal (−) of the comparator 125 is divided voltage atthe input terminal 136, that is, divided from the voltage V1 at thenon-inverted input terminal (+) of the comparator 125 and has a delayedwaveform.

[0086] The resistor 129, the diode 133 and the capacitor 135 work as atime constant circuit (a third time-constant circuit) when the outputvoltage V3 of the comparator 125 changes from the high level to the lowlevel. No time constant circuit is provided when the output of thecomparator 125 changes from the low level to the high level.Consequently, voltage V4 at the output terminal 137 is such that therising of its waveform is substantially equal to the rising of theoutput voltage V3 of the comparator 125 and that the falling of itswaveform is slower than the falling of the output voltage V3 thereof.

[0087] The operation of the switching power supply unit 120 according tothis embodiment of the invention will now be described.

[0088]FIG. 3 is a timing chart showing the operation of the switchingpower supply unit 120 according to this embodiment of the invention.

[0089] As shown in FIG. 3, in the switching power supply unit 120, afirst and a second main switch 107 and 108 are driven alternately underthe control of a control circuit 109 at intervals of predetermined deadtime and in response to the operation, the secondary voltage isgenerated across the source and drain of the second rectifier transistor114 during the interval the first main switch 107 is “on”, whereas thesecondary voltage is generated across the source and drain of the firstrectifier transistor 113 during the interval the second main switch 108is “on”.

[0090] In this case, in the rectifier-transistor driving circuit 104,the first diode 115 is turned on during the interval the first mainswitch 107 is “on” and the second diode 116 is turned on during theinterval the second main switch 108 is “on”. Consequently, during theinterval the first main switch 107 is “on”, the gate-source channel ofthe first rectifier transistor 113 is driven and turned on and duringthe interval the second main switch 108 is “on”, the gate-source channelof the second rectifier transistor 114 is driven and turned on. Further,as the gate of the first rectifier transistor 113 and the gate of thesecond rectifier transistor 114 are short-circuited via the thirdsecondary winding 138 of a transformer 101 during the interval the firstand second main switches 107 and 108 both are “off”, the gate-sourcevoltages of the first rectifier transistor 113 and the second rectifiertransistor 114 each become intermediate voltages.

[0091] As stated above, the voltage Vi supplied to the input terminals136 of the first and second timing generating circuits 121 and 122 has awaveform that repeats three conditions including the low level, the highlevel and an intermediate potential as in the case of Vgs13 or Vgs14 inthe conventional switching power supply unit.

[0092] The operation of the first timing generating circuit 121 will nowbe described.

[0093] As shown in FIG. 3, in such a state that the voltage V1 in thefirst timing generating circuit 121 remains at the low level (beforetime t10), V1<V2, whereby the output voltage V3 of the comparator 125provided in the first timing generating circuit 121 comes to have thelow level. Consequently, the voltage V4 at the output terminal 137 alsohas the low level before the time t10 and the first rectifier transistor113 is held OFF. In the meantime, the capacitor 134 is graduallydischarged via the resistor 128 and the diode 131. In other words, thevoltage V2 lowers at the speed determined by the time constant of thesecond time-constant circuit. In this case, it is necessary for thevoltage V2 to remain lower than the intermediate voltage of the voltageV1 until the arrival of the time t10. Therefore, the time constant ofthe second time-constant circuit is needed to be set so that therelevant conditions are satisfied.

[0094] When the voltage V1 rises from the low level to the intermediatepotential (time t10), V1>V2, whereby the output voltage V3 of thecomparator 125 is inverted and has the high level. When the outputvoltage V3 of the comparator 125 comes to have the high level, thevoltage at the output terminal 137 immediately rises to the high level,whereby the first rectifier transistor 113 is turned on.

[0095] Then the voltage V1 rises from the intermediate potential to thehigh level (time t11) and holds the high level until the timing thefirst main switch 107 is turned off (time t12). In the meantime, thecapacitor 134 is gradually charged via the resistor 126 and the diode130. In other words, the voltage V2 rises at the speed determined by thetime constant of the first time-constant circuit. In this case, it isnecessary for the voltage V2 to rise up to a voltage exceeding theintermediary voltage of the voltage V1 until the arrival of the timet12. Therefore, the time constant of the first time-constant circuit isneeded to be set so that the relevant conditions are satisfied.

[0096] When the voltage V1 falls from the high level to the intermediatepotential (time t12), V1<V2 again, whereby the output voltage V3 of thecomparator 125 is inverted and has the low level. When the outputvoltage V3 of the comparator 125 comes to have the low level, thecapacitor 135 is gradually discharged via the resistor 129 and the diode133. In other word, the voltage V4 falls at the speed determined by thetime constant of the third time-constant circuit.

[0097] With the passage of the predetermined time after the time t12,the voltage V4 at the output terminal 137 falls below the thresholdvoltage Vth113 of the first rectifier transistor 113 (time t13) and thefirst rectifier transistor 113 is turned off. In this case, it isnecessary for the voltage V4 at the output terminal 137 to be lower thanthe threshold voltage Vth113 of the first rectifier transistor 113 priorto the timing the second main switch 108 is turned on (time t14), thatis, the secondary voltage of the transformer 101 is generated across thesource and drain of the first rectifier transistor 113. Therefore, thetime constant of the third time-constant circuit is needed to be set sothat the relevant conditions are satisfied.

[0098] The operation of the second timing generating circuit 122 issimilar to that of the first timing generating circuit 121. At the timet12, the voltage V4 at the output terminal 137 of the second timinggenerating circuit 122 comes to have the high level and after thevoltage V1 falls from the high level to the intermediate potential (timet15), the voltage V4 at the output terminal 137 falls below thethreshold voltage Vth114 of the second rectifier transistor 114 (t16)prior to the timing the first main switch 101 is turned on (time t17),that is, the secondary voltage of the transformer 101 is generatedacross the source and drain of the second rectifier transistor 114.

[0099] Thus, even in consideration of the operational delay unavoidablycaused in the first rectifier transistor 113 and the second rectifiertransistor 114, it is ensured that the first rectifier transistor 113 isturned off at the timing (time t14) the secondary voltage is generatedacross the source and drain of the first rectifier transistor 113 and itis also ensured that the second rectifier transistor 114 is turned offat the timing (time t17) the secondary voltage is generated across thesource and drain of the second rectifier transistor 114. Therefore, nothrough current flows in the first rectifier transistor 113 and thesecond rectifier transistor 114.

[0100] In the switching power supply unit 120 according to thisembodiment of the invention, the first and second timing generatingcircuits 121 and 122 are provided between the rectifier circuit 103 andthe rectifier-transistor driving circuit 104. Accordingly, thegeneration of through current can effectively be prevented by advancingthe timing the first and second rectifier transistors 113 and 114 areturned off without substantially varying the timing the first and secondrectifier transistors 113 and 114 are turned on. Thus, the conversionefficiency of the whole switching power supply unit is enhanced as powerloss is reducible.

[0101] It is further understood that the invention is not limited to thepreferred embodiment of the invention as set forth above but may besubjected to various changes and modifications without departing fromthe scope of the invention and needless to say inclusive of thesechanges and modification as integral part of the claims thereof.

[0102] For example, in the switching power supply unit 120 according tothis embodiment of the invention, though the half-bridge circuit 102 hasbeen employed as the primary circuit of the transformer 101, the primarycircuit of the transformer 101 is not limited to such a half-bridgecircuit but may be any other circuit. FIGS. 4 to 6 show examples ofthose other than the half-bridge circuit.

[0103]FIG. 4 is a circuit diagram of a switching power supply unit 141wherein a full-bridge circuit 140 is used as the primary circuit of thetransformer 101 by way of example. As shown in FIG. 4, it is applicableto the invention to use the full-bridge circuit 140 as the primarycircuit of the transformer 101.

[0104]FIG. 5 is a circuit diagram of a switching power supply unit 143wherein a push-pull circuit 142 is used as the primary circuit of thetransformer 101 by way of example. As shown in FIG. 5, it is applicableto the invention to use the push-pull circuit 142 as the primary circuitof the transformer 101.

[0105]FIG. 6 is a circuit diagram of a switching power supply unit 145wherein an active clamping circuit 144 is used as the primary circuit ofthe transformer 101 by way of example. As shown in FIG. 6, it isapplicable to the invention to use the active clamping circuit 144 asthe primary circuit of the transformer 101.

[0106] With regard to the secondary circuit of the transformer 101,though the rectifier circuit 103 and the smoothing circuit 105 have beenemployed in the switching power supply unit according to this embodimentof the invention, other circuits may be used and FIG. 7 shows an examplein this case. In other words, it is applicable to the invention to usethe circuit shown in FIG. 7 as the secondary circuit of the transformer101.

[0107] The specific circuit configuration of the first and second timinggenerating circuits 121 and 122 according to the above embodiment of theinvention is the one shown by way of example and as long as the on/offtiming of the first and second rectifier transistors 113 and 114 iscontrollable in the same manner as in the above embodiment of theinvention, any timing generating circuit different in configuration maybe used. With regard to the third time-constant circuit including thecapacitor 135, the resistor 129 and the diode 133 provided in each ofthe first and second timing generating circuits 121 and 122, forexample, the capacitor 135 may be deleted by utilizing the capacitanceacross the gate and source of each of the first and second rectifiertransistors 113 and 114.

[0108] A buffer circuit may be inserted in between the gate of each ofthe first and second rectifier transistors 113 and 114 and the outputterminal 137 of each of the first and second timing generating circuits121 and 122. In this case, by setting the threshold voltages of thesebuffer circuits substantially equal to the threshold voltages Vth113 andVth114 of the respective first and second timing generating circuits 121and 122, the gate-source voltage of the first rectifier transistor 113can be set at about 0V at the time t13 and the gate-source voltage ofthe first rectifier transistor 113 can also be set at about 0V at thetime t16.

[0109] (Embodiment 2)

[0110] The second embodiment of the invention will now be described indetail with reference to the drawings, wherein like reference charactersare given to like members in the drawings and the repeated descriptionthereof is omitted. This embodiment of the invention is a modespecifically useful for implementing the invention and the invention isnot limited to the embodiment described herein.

[0111]FIG. 8 is a circuit diagram of a switching power supply unit; FIG.9 is a timing chart showing the voltage/current waveform of each portionin the switching power supply unit of FIG. 8 when the sum of a dead timeperiod and commutation period of a transformer leakage is equal toswitching delay time of a synchronous rectifying element; FIG. 10 is atiming chart showing the voltage/current wave form of each portion whenthe commutation period of the transformer leakage is shorter thanswitching delay time of the synchronous rectifying element in theswitching power supply unit studied by the present inventor; and FIG. 11is a timing chart showing the voltage/current waveform of each portionwhen the commutation period of the transformer leakage is longer thanswitching delay time of the synchronous rectifying element in theswitching power supply unit studied by the present inventor.

[0112] According to this embodiment of the invention, a switching powersupply unit 201 includes a step-down converter (buck converter) 202, ahalf-bridge converter (converter) 203, a control circuit 204, and adriving circuit 205.

[0113] The step-down converter 202 is used for stepping down inputvoltage Vin to a certain voltage level and outputting the stepped-downvoltage. The step-down converter 202 includes a switching element 206, adiode 207, and an inductor 208.

[0114] The half-bridge converter 203 is used for converting the voltagegenerated by the step-down converter 202 to AC voltage once, insulatingand subjecting the AC voltage to voltage conversion, outputting DCoutput voltage Vout having a rectified voltage level and supplying theDC output voltage to a load L.

[0115] Further, the half-bridge converter 203 includes switchingelements 209 and 210, capacitors 211, 212 and 217, a transformer 213,synchronous rectifying switch elements 214 and 215, and an inductor 216.

[0116] The switching elements 206, 209 and 210 and the synchronousrectifying switch elements 214 and 215 are formed of, for example,transistors such as MOS-FETs. The ON/OFF operation of the switchingelement 206 is controlled by the control circuit 204 and the ON/OFFoperation of the switching elements 209 and 210 are controlled by thedriving circuit 205.

[0117] One joint of the switching element 206 is arranged so that inputvoltage Vin is inputted thereto, whereas the other joint of theswitching element 206 is arranged so that the cathode of the diode 207and one joint of the inductor 208 are connected thereto.

[0118] One joint of a capacitor 211 and one joint of the switchingelement (first switching element) 209 are connected to the other jointof the inductor 208.

[0119] One input portion on the primary winding side of the transformer213 and one joint of the switching element (second switching element)210 are connected to the other joint of the switching element 209.

[0120] The other input portion on the primary winding side of thetransformer 213 and one joint of a capacitor (second capacitor ) 212 areconnected to the other joint of the capacitor (first capacitor ) 211.Further, a reference potential (GND) is connected to the anode of thediode 207, the other joint of the switching element 210 and the otherjoint of the capacitor 212.

[0121] A connection is provided so that a control signal from thecontrol circuit 204 is supplied to the control terminal (gate) of theswitching element 206 and a connection is also provided so that acontrol signal (first control signal) OUT1 from the driving circuit 205and a control signal (second control signal) OUT2 are supplied to thecontrol terminals (gates) of the switching elements 209 and 210,respectively.

[0122] One joint of the synchronous rectifying switch element (firstsynchronous rectifying switch element) 214 and the control terminal(gate) of the synchronous rectifying switch element (second synchronousrectifying switch element) 215 are connected to one output portion onthe secondary winding side of the transformer 213.

[0123] One joint of the inductor 216 is connected to one output portion(center tap) on the secondary winding side of the transformer 213. Onejoint of the synchronous rectifying switch element 215 and the controlterminal (gate) of the synchronous rectifying switch element 214 areconnected to the other output portion on the secondary winding side ofthe transformer 213. The other joint of the synchronous rectifyingswitch element 215 is connected to the other joint of the synchronousrectifying switch element 214.

[0124] In the half-bridge converter 203, a half-bridge circuit is formedwith the switching elements 209 and 210 and the capacitors 211 and 212,and a center-tap type output rectifying circuit is formed with thesynchronous rectifying switch elements 214 and 215, the inductor 216 andthe capacitor 217.

[0125] Moreover, the synchronous rectifying switch elements 214 and 215form a self-drive type synchronous rectifier circuit for performingsynchronous rectification by use of the voltage generated on thesecondary winding side of the transformer 213.

[0126] One joint of the capacitor 217 is connected to the other joint ofthe inductor 216, whereas the other joint of the synchronous rectifyingswitch element 215 is connected to the other joint of the capacitor 217.

[0127] The other joint of the inductor 216 and the other joint of thecapacitor 217 constitute the output portion of the switching powersupply unit 201 and the output voltage Vout for being supplied to theload L is sent out of the output portion thereof.

[0128] The control circuit 204 detects the output voltage Vout from thehalf-bridge converter 203 and optimizes the output voltage Vout underits control by making variable the duty ratio of the control signalapplied to the switching element 206 of the step-down converteraccording to the detected results.

[0129] The driving circuit 205 supplies the control signals OUT1 andOUT2 to the control terminals of the respective switching elements 209and 210 provided in the half-bridge converter 203 whereby to control theON/OFF operation of the switching elements 209 and 210. The duties ofthe control signals OUT1 and OUT2 sent out of the driving circuit 205are fixed, so that the switching elements 209 and 210 are each driven tohave dead time.

[0130] The operation of the driving circuit 205 provided in theswitching power supply unit 201 according to this embodiment of theinvention will now be described with reference to FIG. 8 and a signaltiming chart of FIG. 9.

[0131] In FIG. 9, there are shown from top to bottom waveform timing ofthe control signal OUT1 sent out of the driving circuit 205, the controlsignal OUT2 sent out of the driving circuit 205, the voltage V1 acrossboth joints of the synchronous rectifying switch element 214, thevoltage V2 across both joints of the synchronous rectifying switchelement 215, current I1 flowing into the synchronous rectifying switchelement 214, and current I2 flowing into the synchronous rectifyingswitch element 215.

[0132] When the control signal OUT1 is sent out of the driving circuit205, as the switching element 209 is turned on, the voltage V1 on thesecondary winding side of the transformer 213 comes to have the HI leveland the synchronous rectifying switch element 215 is also turned onwhereby to make the current I2 flow.

[0133] In this case, the driving circuit 205 is preset so that thecontrol signals OUT1 and OUT2 containing dead time of t1 shown in FIG. 9are sent out. The control signals OUT1 and OUT2 including the dead timemay be generated by hardware or software.

[0134] In FIG. 9, moreover, time t2 corresponds to delay time in theoperation of the synchronous rectifying switch elements 214 and 215 andtime t3 indicates a commutation period due to leakage from thetransformer 213. FIG. 9 refers to an example in which the delay time t2in the operation of the synchronous rectifying switch element is equalto the sum of the dead time t1 and the commutation period t3.

[0135] The dead time of the control signals OUT1 and OUT2 is set so thatthe sum of the dead time and the commutation period (time t1+time t3) issubstantially equal to the delay time (time t2) in the operation of thesynchronous rectifying switch element or slightly longer than the delaytime t2 in the operation of the synchronous rectifying switch element.

[0136] The most efficient value of the dead time for the generation ofthe switching power supply is given when the sum of the dead time andthe commutation period (time t1+time t3) is equal to the delay time(time t2) in the operation of the synchronous rectifying switch elementand the lower limit value of the dead time is given at this time.

[0137] The upper limit value and the lower limit value are defined bythe following equations.

(lower limit value): t 2≦t 1+t 3  (Condition 1)

(upper limit value): t 1×2<T−t 3×2  (Condition 2)

[0138] Hence, the upper limit value and the lower limit value of thedead time are obtained from the conditions 1 and 2 as follows:

t 2−t 3≦t 1<T/2−t 3

[0139] In this case, T (=1/f) is assumed to be a switching interval (theinterval of the control signal OUT1).

[0140] Further, a case where control signals OUT10 and OUT20 of thedriving circuit studied by the prevent inventor have no dead time willbe described by use of FIGS. 10 and 11.

[0141] In FIGS. 10 and 11, there are shown from top to bottom waveformtiming of the control signal OUT10 sent out of the driving circuit 205provided in the switching power supply unit, the control signal OUT20sent out of the driving circuit 205, voltage V10 across both joints ofthe synchronous rectifying switch element 214, the voltage V20 acrossboth joints of the synchronous rectifying switch element 215, currentI10 flowing into the synchronous rectifying switch element 214, andcurrent I20 flowing into the synchronous rectifying switch element 215.

[0142] In case where no dead time exists in the control signals OUT10and OUT20 sent out of the driving circuit 205, the synchronousrectifying switch elements are simultaneously turned on when delay timein the operation of the synchronous rectifying switch element is longerthan the commutation period due to leakage from the transformer as shownin FIG. 10. Then through current (hatching portions of the currents I10and I20) flows and this may result in not only increasing loss but alsodamaging the synchronous rectifying switch elements when the worst comesto the worst.

[0143] In this case, in order to prevent the through current fromflowing into the synchronous rectifying switch element, the transformeris coarsely coupled so as to increase the commutation period due to theleakage as shown in FIG. 11. However, when the commutation period isprolonged, loss increases as the interval the current flows into thebody diode of the synchronous rectifying switch element increases andmoreover loss due to the leakage from the transformer also increases.

[0144] On the other hand, as it is possible to surely prevent thesynchronous rectifying switch elements 214 and 215 from being damaged aswell as decreasing the commutation period of the transformer 213, thatis, reducing or optimizing leakage inductance in the driving circuit 205for generating the control signals OUT1 and OUT2 having dead timeaccording to this embodiment of the invention, power can be supplied tothe switching power supply unit 201 with efficiency by use of theself-drive type rectifying circuit.

[0145] Thus, reliable, low-cost and low-loss switching power supply unit201 can be supplied by providing dead time for the switching elements209 and 210 according to this embodiment of the invention.

[0146] According to this embodiment of the invention, moreover, though adescription has been given of the switching power supply unit 201adapted for hard switching with respect to the switching elements 209and 210, the switching elements 209 and 210 may be arranged so as tocarry out soft switching by connecting an inductor 218 between the otherjoint of the switching element 209 and one input portion of thetransformer 213 and utilizing dead time as shown in FIG. 12, forexample.

[0147] It is thus possible to materialize low-loss highly-reliableswitching power supply unit.

[0148] Although the step-down converter is provided at the precedingstage according to this embodiment of the invention, a converter to beprovided at the preceding stage may be any converter other than thestep-down converter such as a boost converter and any other converter.

[0149] With regard to the following-state converter, any converter otherthan the half-bridge converter may be employed; for example, variousconverters such as a push-pull or a full-bridge converter are applicableto the invention.

[0150] With regard to the output rectifier circuit, the invention is notlimited to the use of a center tap type but may employ a current doublertype.

[0151] (Embodiment 3)

[0152] A detailed description will now be given of the third embodimentof the invention by reference to the accompanying drawings.

[0153]FIG. 13 is a circuit diagram of a switching power supply unit 330according to the third embodiment of the invention.

[0154] As shown in FIG. 13, a switching power supply unit 330 accordingto this embodiment of the invention is what employs a primary circuitincluding a buck converter circuit and a half-bridge circuit that areconnected in series like a conventional switching power supply unit. Theswitching power supply unit 330 is different from the conventional(third related art) switching power supply unit in that the rectifiercircuit 55 provided in the conventional switching power supply unit isreplaced with a rectifier circuit 331 and that control circuits 332 and333 are provided in place of the control circuit 63 provided in theconventional switching power supply unit. As to the rest, because theswitching power supply unit according to the invention is similar inconfiguration to the conventional switching power supply unit, and thesimilar description thereof will be omitted.

[0155] The rectifier circuit 331 includes a first and a second rectifiertransistor 341 and 342 and the gate of one of the rectifier transistors341 and 342 is connected between the other rectifier transistor and thesecondary winding of a transformer 301. Consequently, on/off of thefirst and second rectifier transistors 341 and 341 is automaticallycontrolled by the voltage generated in the secondary winding of thetransformer 301; that is, the rectifier circuit 331 is a self-drive typesynchronous rectifier circuit.

[0156] The control circuit 332 receives a control signal S given by theuser in addition to output voltage Vo supplied via an insulating circuit308 and forms control pulses Vgs309 and Vgs310 to be supplied to thegate electrodes of a first and a second main switch 309 and 310 providedin a buck converter circuit 303 so that the output voltage Vo has avoltage value shown by the control signal S. In this case, though thecontrol signal S may be a signal capable of choosing two kinds of outputvoltage Vo, 1-bit digital signal is used as the control signal Saccording to this embodiment of the invention. When the control signal Shas a high level, the output voltage Vo (Vo1) should be 3.3V and whenthe control signal S has a low level, the output voltage Vo (Vo2) shouldbe 1.5V. However, the invention is not limited to this arrangement and,for example, three kinds of output voltage Vo may be selected when adigital signal of 302 bits or greater is employed as the control signalS. Further, an analog signal may be used as the control signal S wherebyto show output voltage Vo to be generated linearly according to itsvoltage value (or current value).

[0157] Moreover, control of the control pulses Vgs309 and Vgs310 by thecontrol circuit 332 is so-called duty control and the buck convertercircuit 303 is controlled by adjusting the conducting period of thecontrol pulses in such a condition that the frequencies are fixed.

[0158] On receiving the control signal S, the control circuit 333generates control pulses Vgs311 and Vgs312 to be supplied to the gateelectrodes of a third and a fourth main switch 311 and 312 provided inthe half-bridge circuit 304 according to the control signal S. Morespecifically, the half-bridge circuit 304 is controlled by adjusting thefrequencies according the voltage value indicated by the control signalS in such a condition that dead time DT is fixed.

[0159]FIGS. 14A and 14B are timing charts illustrating the operation ofthe control circuit 333.

[0160] As shown in FIGS. 14A and 14B, the frequencies of the controlpulses Vgs311 and Vgs312 are set to f1 when the control signal S has thehigh level and the frequencies of the control pulses Vgs311 and Vgs312are set to f2 (<f1) when the control signal S has the low level. Whenthe control signal S has either level, however, the period during whicheither of the control pulses Vgs311 and Vgs312 stay at the low level,that is, the dead time DT is so controlled as to be constant.Accordingly, as the duty while the control signal S remains at the lowlevel comes to be lower than the duty while the control signal S remainsat the high level, a step-down level is made larger by the half-bridgecircuit 304 when the control signal S has the low level.

[0161] In this case, how the frequency f1 is set when the control signalS has the high level and how the frequency f2 is set when the controlsignal S has the low level are determined by output voltage Vo1 to begenerated when the control signal S has the high level, output voltageVo2 to be generated when the control signal S has the low level and thefixed dead time DT. It is preferable to determine that the Vo1 to Vo2ratio of the output voltage coincides with the ratio of the duty whenthe control signal S has the high level to the duty when the controlsignal S has the low level. By setting the frequencies f1 and f2 likethis, the step-down level by the buck converter circuit 303 can be madesubstantially constant when the control signal S has either level.

[0162] Further, the dead time DT is preferably determined according todelay time in the operation of the first and second rectifiertransistors 341 and 342 so that the dead time DT substantially conformsto an interval resulting from subtracting a commutation period due toleakage from the transformer 301 from the delay time in the operation ofthe first and second rectifier transistors 341 and 342. The delay timein the operation of the first and second rectifier transistors 341 and342 is defined by duration necessary from the time the gate-sourcevoltage of the first and second rectifier transistors 341 and 342 fallsbelow the threshold value until the first and second rectifiertransistors are actually turned off.

[0163] When the dead time DT is set substantially conformable to aninterval resulting from subtracting a commutation period due to leakagefrom the transformer 301 from the delay time in the operation of thefirst and second rectifier transistors 341 and 342, no through currentflows into the first and second rectifier transistors 341 and 342 andcurrent flowing into the body diodes of the first and second rectifiertransistors 341 and 342 can be decreased as mush as possible.

[0164] At this time, the dead time DT may be set substantially longerthan an interval resulting from subtracting a commutation period due toleakage from the transformer 301 from the delay time in the operation ofthe first and second rectifier transistors 341 and 342 so as to providea margin. In this case, as current flows into the body diodes of therectifier transistors 341 and 342 to the extent of the margin, themargin is preferably kept to a minimum.

[0165] With the arrangement above, in the switching power supply unit330 according to this embodiment of the invention, the control signal Sis caused to have the high level when a load having an operating voltageof 3.3V is driven and the control signal S is caused to have the lowlever when the load having an operating voltage of 1.5V is driven,whereby any load can be driven. In this case, as the switching of theoutput voltage Vo (to 3.3V or 1.5V) is accomplished by switchingoperating frequencies in the half-bridge circuit 304, the load of thebuck converter circuit 303 is prevented from being increased since it isunnecessary to raise the step-down level even when the requested outputvoltage Vo is set low.

[0166] In the switching power supply unit 330 according to thisembodiment of the invention, the switching of the output voltage Vo iscarried out by the switching of the operation of the half-bridge circuitand the output voltage Vo is stabilized through the operation of thebuck converter circuit 303. As the buck converter circuit 303 and thehalf-bridge circuit 304 share their roles with each other, it isunnecessary to coordinate the operation of the buck converter circuit303 with that of the half-bridge circuit closely, so that controlcomplication can be restrained.

[0167] In the switching power supply unit 330 according to thisembodiment of the invention, moreover, the current caused to flow intothe body diodes of the rectifier transistors 341 and 342 can bedecreased as much as possible while the generation of the throughcurrent is prevented by making the dead time DT of the half-bridgecircuit 304 substantially conformable to the interval resulting fromsubtracting the commutation period due to the leakage from thetransformer 301 from the delay time in the operation of the first andsecond rectifier transistors 341 and 342. Thus, the loss produced in therectifier circuit 331 can effectively be suppressed.

[0168] It is further understood that the invention is not limited to thepreferred embodiment of the invention as set forth above but may besubjected to various changes and modifications without departing fromthe scope of the invention and needless to say inclusive of thesechanges and modification as integral part of the claims thereof.

[0169] In the switching power supply unit 330 according to the aboveembodiment of the invention, though the buck converter circuit 303 andthe half-bridge circuit 304 connected in series as the primary circuitof the transformer 301, for example, the invention is not limited to theuse of such a primary circuit applicable to thereto but may be inclusiveof other circuits connected in series. In place of the buck convertercircuit 303, a boost converter circuit, a forward converter circuit, afull-bridge circuit, a push-pull circuit or any other circuit may beemployed, for example.

[0170] In the switching power supply unit 330 according to the aboveembodiment of the invention, further, though the rectifier circuitincluding the first and second rectifier transistors 341 and 342 is usedas the secondary circuit of the transformer 301, the invention islimited to the use of such a secondary circuit of the transformer 1 butmay inclusive of other kinds of circuits, for example, a rectifiercircuit using diodes, for example.

[0171] In the switching power supply unit 330 according to the aboveembodiment of the invention, further, though the 1-bit digital signal isused as the control signal S with the frequencies of the control pulsesVgs311 and Vgs312 set to f1 when the control signal S has the high leveland with the frequencies of the control pulses Vgs311 and Vgs312 set tof2 (>f1) when the control signal S has the low level, a digital signalof 2 bits or greater may be used as the control signal S. In case wherefour kinds of output voltage Vo are selectable, the frequencies of thecontrol pulses Vgs311 and Vgs312 may be set at four correspondingstages. In case where an analog signal is used as the control signal Swith the output voltage Vo selectable linearly according its voltagevalue (or current value), the frequencies of the control pulses Vgs311and Vgs312 may be set linearly.

[0172] As set forth above, it is possible to provide the switching powersupply unit adapted to effectively prevent the generation of throughcurrents according to the invention.

[0173] Further, the invention has the following effect:

[0174] (1) The first and second synchronous rectifying switch elementscan surely be prevented from being simultaneously turned on and theleakage inductance of the transformer is minimized or otherwise thecommutation period due to the leakage inductance is controllable in anoptimum manner, whereby reliable, low-cost and low-loss switching powersupply unit can be provided; and

[0175] (2) As the low-loss switching power supply can be provided, theswitching power supply unit becomes small-sized.

[0176] Furthermore, according to the invention, it is possible toprovide a switching power supply unit wherein switching of the outputvoltage Vo can be carried out under a simple type of control and anincrease in the loss generated in the rectifier circuit is suppressed.

What is claimed is:
 1. A switching power supply unit comprising: atransformer; a switching circuit provided on the primary side of thetransformer; a synchronous rectifier circuit provided on the secondaryside of the transformer and having at least a rectifier transistor; arectifier-transistor driving circuit provided on the secondary side ofthe transformer and forming a first control signal synchronous with aswitching operation of the switching circuit; and a timing generatingcircuit which receives the first control signal for forming a secondcontrol signal which exceeds a threshold voltage of the rectifiertransistor at a timing substantially equal to the timing that one edgeof the first control signal is generated based on the first controlsignal and which falls below the threshold voltage of the rectifiertransistor at a timing earlier by predetermined time than the timing theother edge of the first control signal is generated, and for supplyingthe resulting second control signal to the control electrode of therectifier transistor.
 2. The switching power supply unit as claimed inclaim 1, wherein the waveform of the first control signal is a waveformalternately repeating a first potential, a second potential and anintermediate potential between the first and second potentials, whereinthe one edge of the first control signal is defined by the timing theone edge varies from the first potential to the intermediate potential,and the other edge of the first control signal is defined by the timingthe other edge varies from the intermediate potential to the firstpotential.
 3. The switching power supply unit as claimed in claim 2,wherein during the interval the first control signal varies from theintermediate potential to the first potential after the first controlsignal varies from the second potential to the intermediate potential,the voltage of the second control signal falls below the thresholdvoltage of the rectifier transistor.
 4. The switching power supply unitas claimed in claim 3, wherein the timing generating circuit including:a first unit which receives the first control unit for forming anintermediate signal which varies from a first logical level to a secondlogical level in response to the one edge of the first control signaland varies from the second logical level to the first logical level inresponse to the variation of the first control signal from the secondpotential to the intermediate potential; and a second unit whichreceives the intermediate signal for forming the second control signalby providing a delay to the variation of the intermediate signal fromthe second logical level to the first logical level.
 5. The switchingpower supply unit as claimed in claim 4, wherein the first unitincluding: a divider circuit for dividing the first control signal; adelay circuit for delaying the output signal of the divider circuit; anda comparator for comparing the first control signal with the outputsignal of the delay circuit whereby to form the intermediate signal. 6.The switching power supply unit as claimed in claim 5, wherein the delaycircuit including: a first time-constant circuit for providing a delayto the one-directional variation of the output signal of the dividercircuit; and a second time-constant circuit for providing a delay to thereverse-directional variation of the output signal of the dividercircuit.
 7. The switching power supply unit as claimed in claim 6,wherein the time constant of the first time-constant circuit is set sothat the potential of the output signal of the delay circuit rises aboveat least the intermediate potential at the timing that the first controlsignal varies from the second potential to the intermediate potentialwherein the time constant of the second time-constant circuit is set sothat the potential of the output signal of the delay circuit falls belowat least the intermediate potential at the timing the first edge of thefirst control signal is generated.
 8. The switching power supply unit asclaimed in claim 1, wherein the switching circuit is one selected from ahalf-bridge circuit, a full-bridge circuit, a push-pull circuit and anactive clamping circuit.
 9. A switching power supply unit comprising: aswitching circuit which is connected to an input power supply and has afirst main switch and a second main switch which alternately conduct atintervals of dead time; a first rectifier transistor for performingrectifying operation during the interval that the second main switchremains non-conducting; a second rectifier transistor for performingrectifying operation during the interval the first main switch remainsnon-conducting; and a driving unit for driving the first and secondrectifier transistors, wherein the driving unit is used to supply an ONsignal to the control electrode of the first rectifier transistor overthe substantially whole period of first dead time to be inserted andalso to supply the ON signal to the electrode of the second rectifiertransistor for a part of period of the first dead time when theconducting main switch is switched from the second main switch to thefirst main switch, wherein the driving unit is used to supply the ONsignal to the control electrode of the second rectifier transistor overthe substantially whole period of second dead time to be inserted andalso to supply the ON signal to the electrode of the first rectifiertransistor for a part of period of the second dead time when theconducting main switch is switched from the first main switch to thesecond main switch.
 10. The switching power supply unit as claimed inclaim 9, wherein a part of period of the first dead time is aconsecutive period including the timing of starting the first dead time,wherein a part of period of the second dead time is a consecutive periodincluding the timing of starting the second dead time.
 11. A switchingpower supply unit comprising: a first switching element and a secondswitching element which are provided on the primary winding side of atransformer and connected to a power supply in series; a converterhaving a first synchronous rectifying switch element and a secondsynchronous rectifying switch element which are connected to thesecondary winding side of the transformer in series; and a drivingcircuit for controlling the operation of the first and second switchingelements and generating a first control signal and a second controlsignal both having a dead time period in which the first and secondswitching elements are not conducting.
 12. The switching power supplyunit as claimed in claim 11, wherein the dead time of the first andsecond control signals generated by the driving circuit is a period orlonger resulting from subtracting a commutation period due to leakageinductance from the transformer from delay time in the operation of thesynchronous rectifying switch element and is a period shorter than timeresulting subtracting the commutation period from a half period of thefirst control signal.
 13. The switching power supply unit as claimed inclaim 11, wherein an inductor is provided in one input portion on theprimary winding side of the transformer, the inductor being used forcausing the first and second switching elements to carry out softswitching during the dead time period.
 14. The switching power supplyunit as claimed in claim 11, wherein a preceding-stage converterincluding at least one switching element is provided at the precedingstage of the converter.
 15. The switching power supply unit as claimedin claim 14, wherein the converter has a fixed duty and thepreceding-stage converter performs pulse width control.
 16. Theswitching power supply unit as claimed in claim 11, wherein theconverter is a half-bridge converter.
 17. The switching power supplyunit as claimed in claim 16, wherein the half-bridge converter includes:a half-bridge circuit having a first capacitor and a second capacitorprovided on the primary winding side of the transformer and a firstswitching element and a second switching element which are connected tothe power supply in series; and a self-drive type synchronous rectifyingcircuit having a first and a second synchronous rectifying switchelement connected to the secondary winding side of the transformer inseries.
 18. A switching power supply unit comprising: a transformer; afirst converter and a second converter which are connected between asupply input terminal and the primary winding of the transformer inseries; an output circuit connected to the secondary winding of thetransformer; and a control circuit for controlling the operation of thefirst and second converters, wherein the control circuit is used forcontrolling the first converter in terms of duty and also controllingthe second converter in terms of frequency.
 19. The switching powersupply unit as claimed in claim 18, wherein the control circuit controlsthe duty of the first converter according to a present output voltageoutputted from the output circuit, and controls the operating frequencyof the second converter according to a set value regardless of thepresent output voltage.
 20. The switching power supply unit as claimedin claim 18, wherein the control circuit controls the second converterso that dead time is made constant, regardless of the operatingfrequency.
 21. The switching power supply unit as claimed in claim 20,wherein the output circuit includes a self-drive type synchronousrectifier circuit formed with rectifier transistors is contained,wherein the dead time is set substantially equal to an intervalresulting from subtracting a commutation period due to leakageinductance from the transformer from delay time in the operation of therectifier transistors.
 22. The switching power supply unit as claimed inclaim 18, wherein the first converter is a buck converter circuit andthe second converter is a half-bridge converter circuit.